The present invention comprises a cyclone-type particle classifier which contains a foraminous inner cone contained within a solid-walled outer cone. An air gap is present between the two cones in the area where they join the main body of the cyclone. The apparatus lacks the usual top exhaust for particle-free air.
Cyclone separators for removing solid materials from a gas stream have been available for approximately the last century. Within 15 years of their introduction into the engineering community, designs were available which served not only a separating function, but a particle classifying function as well. Examples are seen in U.S. Pat. Nos. 687,226 to Raymond, and 1,165,866 to Fraser.
The art has developed significantly from that early time. One approach to using a cyclone as a classifier involves the use of an inner foraminous cone within an outer solid cone. In most cases, the coarse material is retained within the annular space between the two cones, while the finer material passes through the foraminae and is exhausted from the top of the cylone. Apparatus of this type is shown in U.S. Pat. No. 3,513,642 to Cornett. A similar apparatus, in which two streams are exhausted from the bottom of the cyclone, is shown in U.S. Pat. No. 3,341,011 to Prescott. Other examples of cyclone-type classifiers are seen in U.S. Pat. Nos. 3,615,008 to Alpha, and 3,667,600 to Oi et al.
Shumate (Proceedings, Particleboard Symposium, Washington State University, 5:243-261, 1971) gives an engineering and historical summary of various types of cyclone separators. In this paper he states, "Numerous attempts have been made to make a cyclone act as a classifier by installing an inner cone of perforated metal or woven wire. These have met with indifferent success, depending upon the material being screened."
One commercial unit containing a perforated inner core is in use primarily for screening heavy trash from pulp chips. This unit has apparently been commercially successful because of the relatively large difference in size and specific gravity between the accepted and rejected fractions. However, as Shumate notes, classifiers based on cyclones have been far less successful where the size and specific gravity differences between accepted and rejected portions are small or nonexistent.